Subject: Silence Supression - Frame Based Speech Codecs?

From: Rafiq Shaikh <[email-masked]>
Date: Thu, 21 Oct 1999 15:51:06 +0530 (IST)

Hi all,

As far as RTP is concerened, how is Silence Supression implemented w.r.t.
Frame Based speech codecs?

During Silence supression, are RTP packets sent without any Payload or
we don't need to send RTP packets at all?

If No RTP packet is sent for the Silence period:
 a) How do we distinguish a Packet loss with No packet sent? 
   (During Silence frames)
 b) In RTP, is there a restriction on the duration of Silence period?
 c) What is the possible effect on the statistical calculations done in
    the RTCP part.

How can we utilize the Marker bit in the RTP Header to distinguish 
between the NULL frame and the Active frame within the same RTP Packet?

Thanks in advance.

Regards,
-Rafiq.

---

From: [email-masked]
Date: Thu, 21 Oct 1999 10:49:11 -0400

During Silence supression, are RTP packets sent without any Payload or
we don't need to send RTP packets at all?
Answer:  I believe you do not need to send any packets at all. This is
vocoder specific, however, all of
the ones which I am familiar with send special packets at the beginning of
a silence period and periodically
during one (at a very reduced rate).

If No RTP packet is sent for the Silence period:
 a) How do we distinguish a Packet loss with No packet sent?
   (During Silence frames)
Answer:  Excellent question.  I believe that you cannot tell.  There are
some augmentations in the works for
RTP such as redundancy which attempt to grapple with this.
 b) In RTP, is there a restriction on the duration of Silence period?
Answer:  As far as I know, RTP itself imposes no restriction.
 c) What is the possible effect on the statistical calculations done in
    the RTCP part.
Answer:  I believe there is no major effect on the calculation of jitter.
Packet loss however does have
an effect.

How can we utilize the Marker bit in the RTP Header to distinguish
between the NULL frame and the Active frame within the same RTP Packet?
Answer:  Again this is vocoder specific.  Some vocoders have rules about
what sort of frame can follow another frame.  Others have implicit length
indicators within each frame.  The marker bit is essentially
useless/redundant in my humble opinion.

Hope that helps.
Other replies invited.